Controller yaw authority still isn't as good as it could be, so in the course of my experimentation I decided to try out shorter arms (less rotational inertia). Here's a view of the whole contraption sitting on the table. I moved the landing gear inward and attached pool noodle for padding. Nice, but this arrangement was very tippy on landing if there was any lateral motion at all. I didn't want to take the time to add little legs to the ends of the arms, so I inserted some square carbon fiber tubes that I had laying around into the pool noodle. Worked like a charm. Adds weight, but I'll worry about that after I've truly solved the heading hold issue: